Fix port conflict: separate Next.js (3001) from Go API (PORT)

- Next.js admin now runs on internal port 3001
- Go API uses Dokku's PORT environment variable (5000)
- Updated admin proxy default to localhost:3001
- Added /_next/* static asset proxy route

This fixes the "address already in use" error when deploying to Dokku
where both services were trying to bind to port 3000.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/admin/routes.go

@@ -359,10 +359,11 @@ func SetupRoutes(router *gin.Engine, db *gorm.DB, cfg *config.Config, deps *Depe
 
 // setupAdminProxy configures reverse proxy to the Next.js admin panel
 func setupAdminProxy(router *gin.Engine) {
-	// Get admin panel URL from env, default to localhost:3000
+	// Get admin panel URL from env, default to localhost:3001
+	// Note: In production (Dokku), Next.js runs on internal port 3001
 	adminURL := os.Getenv("ADMIN_PANEL_URL")
 	if adminURL == "" {
-		adminURL = "http://127.0.0.1:3000"
+		adminURL = "http://127.0.0.1:3001"
 	}
 
 	target, err := url.Parse(adminURL)
@@ -381,4 +382,9 @@ func setupAdminProxy(router *gin.Engine) {
 	router.Any("/admin", func(c *gin.Context) {
 		c.Redirect(http.StatusMovedPermanently, "/admin/")
 	})
+
+	// Proxy Next.js static assets
+	router.Any("/_next/*filepath", func(c *gin.Context) {
+		proxy.ServeHTTP(c.Writer, c.Request)
+	})
 }